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1978). IFIM combines two models, a biological one that describes the physi- cal habitat preferences of fishes (and occasionally macroinvertebrates) in terms of depth, velocity, and substrate, and a hydraulic one that estimates how the availability of habitat for fish varies with discharge. IFIM has been widely used as an organiza- tional framework for formulating and evaluating alternative water management options related to pro- duction of one or a few fish species (Stalnaker et al. 1995). As a predictive tool for ecological management, the IFIM modeling approach has been criticized both in terms of the statistical validity of its physical habitat characterizations (Williams 1996) and the limited re- alism of its biological assumptions (Castleberry et al. 1996). Field tests of its predictions have yielded mixed results (Morehardt 1986). Although this approach continues to evolve, both by adding biological realism (Van Winkle et al. 1993) and by expanding the range of habitats modeled (Stalnaker et al. 1995), in practice it is often used only to estab- lish minimum flows for "important" (i.e., game or imperiled) fish species. But current understanding of river ecology clearly indicates that fish and other aquatic organisms require habitat features that cannot be main- tained by minimum flows alone (see Stalnaker 1990). A range of flows is necessary to scour and revitalize gravel beds, to import wood and organic matter from the floodplain, and to provide access to productive riparian wetlands (Figure 4). Inter- annual variation in these flow peaks is also critical for maintaining chan- nel and riparian dynamics. For ex- ample, imposition of only a fixed high-flow level each year would sim- ply result in the equilibration of in- channel and floodplain habitats to these constant peak flows. Moreover, a focus on one or a few species and on minimum flows fails to recognize that what is "good" for the ecosystem may not consistently benefit individual species, and that what is good for individual species may not be of benefit to the ecosys- tem. Long-term studies of naturally variable systems show that some spe- cies do best in wet years, that other species do best in dry years, and that overall biological diversity and eco- system function benefit from these variations in species success (Tilman et al. 1994). Indeed, experience in river restoration clearly shows the impossibility of simultaneously en- gineering optimal conditions for all species (Sparks 1992, 1995, Toth 1995). A holistic view that attempts to restore natural variability in eco- logical processes and'species success (and that acknowledges the tremen- dous uncertainty that is inherent in attempting to mechanistically model all species in the ecosystem) is neces- sary for ecosystem management and restoration (Franklin 1993). Managing toward a natural flow regime The first step toward better incorpo- rating flow regime into the manage- ment of river ecosystems is to recog- nize that extensive human alteration of river flow has resulted in wide- spread geomorphic and ecological changes in these ecosystems. The his- tory of river use is also a history of flow alteration (Figure 5). The early establishment of the US Army Corps of Engineers is testimony to the im- portance that the nation gave to de- veloping navigable water routes and to controlling recurrent large floods. However, growing understanding of the ecological impacts of flow alter- ation has led to a shift toward an appreciation of the merits of free- flowing rivers. For example, the Wild and Scenic Rivers Act of 1968 recog- nized that the flow of certain rivers should be protected as a national resource, and the recent blossoming of natural flow restoration projects (Table 3) may herald the beginning of efforts to undo some of the dam- age of past flow alterations. The next century holds promise as an era for renegotiating human relationships with rivers, in which lessons from past experience are used to direct wise and informed action in the future. A large body of evidence has shown that the natural flow regime of virtually all rivers is inherently variable, and that this variability is critical to ecosystem function and native biodiversity. As we have al- ready discussed, rivers with highly altered and regulated flows lose their ability to support natural processes and native species. Thus, to protect pristine or nearly pristine systems, it is necessary to preserve the natural hydrologic cycle by safeguarding against upstream river development and damaging land uses that modify runoff and sediment supply in the watershed. Most rivers are highly modified, of course, and so the greatest chal- lenges lie in managing and restoring rivers that are also used to satisfy human needs. Can reestablishing the natural flow regime serve as a useful management and restoration goal? We believe that it can, although to varying degrees, depending on the present extent of human interven- tion and flow alteration affecting a particular river. Recognizing the natural variability of river flow and explicitly incorporating the five com- ponents of the natural flow regime (i.e., magnitude, frequency, duration, timing, and rate of change) into a broader framework for ecosystem management would constitute a major advance over most present management, which focuses on mini- mum flows and on just a few species. Such recognition would also con- tribute to the developing science of stream restoration in heavily altered watersheds, where, all too often, physical channel features (e.g., bars and woody debris) are re-created without regard to restoring the flow regime that will help to maintain these re-created features. Just as rivers have been incremen- tally modified, they can be incre- mentally restored, with resulting improvements to many physical and biological processes. A list of recent efforts to restore various components of a natural flow regime (that is, to "naturalize" river flow) demon- strates the scope for success (Table 3). Many of the projects summarized in Table 3 represent only partial steps toward full flow restoration, but they have had demonstrable ecological benefits. For example, high flood flows followed by mimicked natural rates of flow decline in the Oldman River of Alberta, Canada, resulted in a massive cottonwood recruitment that extended for more than 500 km downstream from the Oldman Dam. Dampening of the unnatural flow fluctuations caused by hydroelectric generation on the Roanoke River in 780 BioScience Vol. 47 No. 11